This invention relates to controlled rectifier circuits for supplying direct current to a motor or other type of load from an alternating source, and more particularly to automatic control for the rectifier circuit to limit acceleration of the motor. DC motors can be operated with single phase AC current by converting the AC current to DC current through a full-wave diode bridge rectifier. Once power is supplied, the motor receives full power. The speed of the motor with full power is determined only by its design.
A wide variety of silicon-controlled-rectifier (SCR) circuits have been developed in order to obtain more control over DC motors than simple on/off control. SCR circuits are designed around full-wave, or half-wave, rectifiers to control motor speed by controlling the SCR firing angle. Generally feedback circuits compensate for motor losses and maintain constant speed over a wide range of load conditions. Such SCR circuits are not suitable for applications having only two modes of operation, namely power off and full power on, due to the complexity of the feedback circuits not necessary if full power is to be applied all the time once the motor has reached full operating speed. Examples of applications requiring only full power when on are pumps, conveyors, fans, vacuum sweepers and the like, where speed regulation for varying load conditions is not required, and instead full design speed of the motor is desired.
If a permanent-magnet motor is provided with only on/off control there is a problem in that during acceleration immediately after applying power, the DC current reaches a high level. In most permanent magnet motor designs, this current can partially demagnetize the field magnets. Another problem is that a permanent-magnet motor has high starting torque and could cause damage to the load, such as damage to the fan and brush in a vacuum sweeper. To avoid such damage and/or partial demagnetization of the field magnets, it is desirable to control the power applied to the motor during acceleration of the motor to its designed speed.